I think you'll find the majority of the guys around here use either LoadBase or Exbal. LoadBase runs under the Windows platform. Exbal has a version for Windows and another version for Palms.
There are several software packages for the Iphone/Itouch and my hunting buddy uses Bullet Flite on his ITouch and likes it just fine. I usually use LoadBase on a Dell Axim in the field, plus on my PC at home. I also use Shooter on my HTC Evo Android and it's also a great program but when it gets right down to it I pull out the Dell Axim with LoadBase on it.
Most of them get the job done, especially at shorter ranges but you may find that one or two work better for you at longer ranges and under widely varied conditions.
Whatever you use just remember that the data you put in has to be correct and the way you use the software can determine whether or not it works satisfactorily for you. Use the search feature and you'll find a ton of information.